Fundamentals of Pharmacology for Veterinary Technicians

1. Fundamentals of Pharmacology for Veterinary Technicians Chapter 15 Antiparasitics © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

2. Basic Terminology • Endoparasites live within the body of the host and cause internal parasite infections • Ectoparasites live on the body surface of the host and cause external parasite infestations © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

3. Endoparasites • Helminths are divided into two major groups: • Nematodes: cylindrical, nonsegmented worms commonly called roundworms • Platyhelminths: flattened worms that are subdivided into two groups: • Cestodes (tapeworms) • Trematodes (flukes) • Anthelmintics kill worm parasites • Antinematodals • Anticestodals • Antitrematodals © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

4. Antinematodals • Benzimidazoles: work by interfering with energy metabolism of the worm • Always read the label to determine which parasites the drug is effective against • Recognize by –azole ending in drug name • Thiabendazole: has antifungal and anti-inflammatory effects as well • Oxibendazole: liver toxicity in dogs • Mebendazole: liver toxicity issues • Fenbendazole: wide spectrum of activity; given for three consecutive days • Febantel: a probenzimidazole that is metabolized in the animal to a true benzimidazole © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

5. Antinematodals • Imidazothiazoles • Work by stimulating the nematode’s cholinergic nervous system, leading to paralysis of the parasite (therefore, not ovicidal) • Effective against ascarids, strongyles, whipworms, and hookworms • An example is levamisole • Tetrahydropyrimidines • Mimic the action of ACh and cause paralysis of the worm • Effective against ascarids, pinworms, strongyles, and hookworms • Examples include pyrantel pamoate, pyrantel tartrate, and morantel tartrate © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

6. Antinematodals • Organophosphates • Inhibit cholinesterase activity, causing ACh to remain active in the neuromuscular junction of the parasite • Are neurotoxic to parasites; some cause neurologic side effects in the host • Both endoparasitic and ectoparasitic • Narrow range of safety; not for use in heartworm-positive dogs • Effective against bots and a variety of nematodes • Examples include dichlorvos and coumaphos • Piperazine • Blocks neuromuscular transmission in the parasite • Effective only against ascarids © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

7. Antinematodals • Avermectins (macrocyclic lactones) • Bind to certain chloride channels in the parasite nerve and muscle cells, causing paralysis and death of the parasite • The representative of this group is ivermectin, used for a wide variety of endo- and ectoparasites • May be combined with other antiparasitic agents to broaden its spectrum of activity • Used for heartworm prevention • Another example in this group is moxidectin • Not effective against cestodes or trematodes © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

8. Anticestodals • Praziquantel • Works by increasing the cestode’s cell membrane permeability (this disintegrates the worm’s outer tissue covering) • Works on all cestode species (also used to eliminate fleas) • Epsiprantel • Causes disintegration of the cestode • Effective against Taenia and Dipylidium, but not Echinococcus • Fenbendazole • Covered previously • Effective against Taenia species © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

9. Antitrematodals • Clorsulon • Works by inhibiting the trematode’s enzyme systems for energy production • Effective against Fasciola hepatica • Albendazole • Interferes with the energy metabolism of the worm (also effective against some nematodes) • Praziquantel • Covered previously • Also effective against lung trematodes in dogs and cats © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

10. Anticoccidials • Coccidiosis is a protozoal infection that causes intestinal disorders • Anticoccidial drugs are coccidiostats (do not actually kill the parasite, so hygiene is crucial) • Sulfadimethoxine • Reduces the number of oocysts shed, thus reducing spread of disease • Others (work mainly by affecting the protozoan’s metabolism) • Nicarbazine • Amprolium • Monensin • Decoquinate • Robenidine © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

11. Antiprotozoals • Giardiosis is a protozoal disease caused by the parasite Giardia lamblia • Antiprotozoal drugs • Metronidazole (enters the protozoal cell and interferes with its ability to function and replicate) • Fenbendazole (covered previously) • Albendazole (covered previously) • Vaccine • Blood protozoan Babesia sp. is transmitted by ticks • Imidocarb has cholinergic effects on the protozoan • Tick prevention © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

12. Treatment of Heartworm Disease • Heartworm disease is caused by the filarial nematode Dirofilaria immitis • Three stages of management of heartworm disease • Preventing third-stage larvae from reaching maturity (preventative) • Adulticide therapy • Eradication of circulating microfilariae after infection © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

13. Treatment of Heartworm Disease • Preventing third-stage larvae from reaching maturity (preventative) • Daily oral preventative • Diethylcarbamazine (DEC) • Given during mosquito season and two months after • Patient must be heartworm negative • Once-monthly oral preventatives • Ivermectin • Milbemycin • Once-monthly topical preventative • Selamectin • Six-month injectable preventative • Moxidectin © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

14. Treatment of Heartworm Disease • Adulticide therapy • Melarsomine • Given in the epaxial muscles • Less toxic than former drug (thiacetarsamide) • Side effects include nephrotoxicity and hepatotoxicity • Eradication of circulating microfilariae after infection • Ivermectin (given at higher dose as a microfilaricide) • Milbemycin • Levamisole (infrequently used) © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

15. Ectoparasite Treatment • Ectoparasites can be controlled using a variety of different drugs in a variety of different formulations • Sprays • Dips • Pour-ons • Shampoos • Dusts or powders • Foggers • Oral products • Spot-ons • Injectables • See Table 15-4 for forms of ectoparasites and their advantages/disadvantages © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

16. Chemicals Used for Ectoparasite Treatment • The chemicals used in ectoparasite treatment are summarized in Table 15-5 • Always read product labels to determine what safety procedures to follow • May need protective clothing • May need special disposal techniques • Proper ventilation is crucial • Keep and refer to MSDS prior to use and if signs of toxicity occur in the animal © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.

17. Pyrethrins and pyrethroids Insect growth regulators Chitin synthesis inhibitors Neonicotinoid Carbamates OPs Formamidines Synergists Imidacloprid Imidacloprid + permethrin Lime sulfur Fipronil Repellents Rotenone Ivermectin Selamectin D-limonene Chemicals Used for Ectoparasite Treatment © 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.